SPARK-2461. Add a toString method to GeneralizedLinearModel

it's better to use __repr__() here, so you can better string representation when it's inside REPR or in another object(such as tuple). Also, The default __str__() will call __repr__()

```
>>> class C(object):
...    def __str__(self):
...        return 'str'
...    def __repr__(self):
...        return 'repr'
...
>>> print c
str
>>> c = C()
>>> c
repr
>>> (c, c)
(repr, repr)
>>> print tuple((c, c))
(repr, repr)
```